Sheet folding apparatus and sheet post-processing apparatus

A sheet folding apparatus which folds a sheet of paper in two and comprises:

• a couple of two-folding rollers each of whose outer diameter gradually decreases from the central part to each end part in a longitudinal direction thereof;
• a folding plate which can move forward to or backward from a nip between the two-folding rollers, and has a cutout part which is located at an edge part of the folding plate facing the nip between central parts of the two-folding rollers, wherein the cutout part does not reach the nip between the central parts of the two-folding rollers when the sheet is folded;
• a drive mechanism which moves the folding plate forward;
• a folding plate support member which is placed at a position facing the nip between the central parts of the two-folding rollers, is slidably supported on the folding plate so as to be able to slide forward or backward with respect to the folding plate, and is always pushed forward by receiving a pushing force such that an end of the folding plate support member projects from the cutout part;
• and
• a catching member which moves the folding plate support member backward with respect to the folding plate against the pushing force by touching the folding plate support member, after the folding plate support member starts to move forward to the nip together with the folding plate by driving the drive mechanism and before the folding plate support member reaches the nip.